Wireless portable computer capable of autonomously adjusting load of wireless base station

ABSTRACT

A method for allowing a portable computer to adjust the load of a wireless base station autonomously is disclosed. The portable computer can communicate in an infrastructure mode and an ad-hoc mode. The portable computer includes a traffic table for storing the performance of various wireless stations and the traffic of other portable computers. Each portable computer measures its traffic to a wireless base station. Then, the portable computer transmits the measured traffic to the other portable computers in the ad-hoc mode and receives the traffic of the other portable computers. A newly connected portable computer acquires the traffic in the ad-hoc mode. When the newly connected portable computer enters into a cell overlap area to connect to any wireless base station, the newly connected portable computer selects a wireless base station having little traffic based on the performance and traffic of wireless base stations instead of a wireless base station that provides a stronger radio field.

PRIORITY CLAIM

The present application claims benefit of priority under 35 U.S.C.§§120, 365 to the previously filed Japanese Patent Application No.JP2010-088300 entitled, “Wireless terminal capable of autonomouslyadjusting load of wireless base station” with a priority date of Apr. 7,2010, which is incorporated by reference herein.

BACKGROUND

1. Technical Field

The present invention relates to portable computers in general, and inparticular to a method for adjusting the load of a wireless basestation. More particularly, the present invention relates to a methodfor allowing a wireless portable computer to adjust the load of awireless base station autonomously.

2. Description of Related Art

Wireless terminals such as notebook-type portable computers (laptop PCs)or personal digital assistants (PDAs) have a wireless LAN communicationdevice mounted therein and are capable of communicating with a server orother laptop PCs through a wireless base station. Moreover, in awireless LAN network, multiple wireless base stations is disposed sothat the ranges of radio coverage partially overlap with each other. Thewireless terminal has a fallback function that automatically decreases atransmission rate so that the communication is not disconnected in orderto perform signal processing of noise separated from the signals whenthe radio field intensity is weak. Thus, when multiple wireless basestations is detected to be connectable, the wireless terminal isconfigured to automatically connect to a wireless base station whichprovides the highest radio field intensity and from which a hightransmission rate can be expected. Therefore, in a building where thewireless LAN network is installed, when many users carrying theirwireless terminals gather in a certain space such as a meeting room, theconnection is concentrated on a specific wireless base station whichprovides a strong radio field.

The number of wireless terminals connectable to one wireless basestation is generally limited by the number of IP addresses assignedthrough the wireless base station. Although the IP addresses areautomatically assigned by a DHCP server, a network administrator limitsthe number of IP addresses handled by a specific wireless base stationfrom the perspective of managing security or the like. Therefore, thenumber of host addresses that are actually handled by a wireless basestation is far smaller than the theoretical number defined in the globaladdress class. Moreover, in some cases, the network administrator limitsthe number of wireless terminals connected to a specific wireless basestation at the same time in order to maintain throughput. Since the IEEE802.11 standards have no regulations on the time when roaming ordisconnection occurs, a wireless terminal having connected to a specificwireless base station generally tends to maintain the connection unlessit moves or is suspended. As a result, a number of wireless terminalswhich do not actually transmit data are connected to a specific wirelessbase station. Therefore, a wireless terminal which needs to transmitdata is unable to connect to the specific wireless base station.

Moreover, even when multiple wireless terminals is connected to aspecific wireless base station, only one wireless terminal can actuallycommunicate with the wireless base station at a certain point in time.In order to avoid a collision of frames, the other wireless terminalsperform carrier sensing and wait until a wireless channel enters an idlestate and then transmit data. Therefore, if the number of wirelessterminals connected to a specific wireless base station increases or thetransmission of data from the respective wireless terminals isconcentrated at a certain point in time, the overall transmission rateof all the wireless terminals connected to the wireless base stationdecreases. In addition, in the case of a collision of frames, thewireless terminal is unable to receive an ACK frame representing anacknowledgement from the wireless base station, the wireless terminalhas to transmit the same data again, which further decreases thetransmission rate.

As described above, when a number of wireless terminals connected to aspecific wireless base station perform data transmission at the sametime, the traffic (unit: bps) which is the quantity of data transmittedper unit period by the respective wireless terminals decreases. Thus, itbecomes impossible to perform comfortable wireless communication. In awireless base station, the throughput (unit: bps) which represents thequantity of data transmitted per unit period is set. For the respectivewireless terminal to maintain a predetermined range of transmissionrates, it is preferable to adjust the total traffic so that thethroughput of the wireless base station has a predetermined margin withrespect to the total traffic of the respective wireless terminal.

SUMMARY

In accordance with a preferred embodiment of the present invention, alaptop computer initially acquires performance information representinga performance of multiple wireless base stations that form a celloverlap area to which multiple wireless terminals is connected. Thelaptop computer is capable of communicating in an infrastructure modeand an ad-hoc mode to allow the laptop computer to connect to any one ofthe wireless base stations. Before connecting to any one of the wirelessbase stations in the cell overlap area, the laptop computer communicateswith the wireless terminals in the ad-hoc mode to receive connectioninformation representing respective connection states of the wirelessterminals to the wireless base stations. Based on the performanceinformation and the connection information, the laptop computer selectsone of the wireless base station as a candidate for making a wirelessconnection. The laptop computer subsequently connects to the selectedwireless base station.

All features and advantages of the present invention will becomeapparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself, as well as a preferred mode of use, furtherobjects, and advantages thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment whenread in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram showing a network configuration of an infrastructuremode wireless LAN system;

FIG. 2 is a diagram showing a network configuration of an ad-hoc modewireless LAN system;

FIG. 3 is a block diagram showing a hardware configuration of a station;

FIG. 4 is a diagram showing the software and hardware configuration forallowing each station to autonomously adjust the load of an accesspoint;

FIG. 5 is a diagram showing a data structure of a traffic table;

FIG. 6 is a diagram illustrating a method for determining averagetraffic from measured traffic;

FIG. 7 is a flowchart showing a method of a station determining its ownconnecting destination when newly participating in a basic service setand adjusting the load of an access point; and

FIG. 8 is a flowchart showing the procedure in which a STA havingalready participated in a basic service set autonomously performsroaming and adjusts the load of an access point.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT A. Configuration ofWireless LAN System

FIG. 1 is a diagram showing a network configuration of a wireless LANsystem 10 employing an infrastructure mode (infra mode) defined by theIEEE 802.11 standards. FIG. 1 shows a representative example in which anauthentication server 13, a wireless base station or an access point(AP) 20, and an AP 30 are connected to a backbone network 11 of theEthernet. However, actually, various servers such as a Web server, amail server, a DNS server, and a database server are connected to thebackbone network 11. An area defined as the range of radio coverageserved by one AP is called a cell. FIG. 1 shows a cell 21 of the AP 20and a cell 31 of the AP 30. The cells 21 and 31 include an overlap area.An area in which cells overlap one another is called a cell overlaparea. When three or more APs are disposed, the cell overlap area may beformed by three or more cells. Moreover, an area in which the celloverlap area is excluded from each cell is called a cell exclusive area.

The wireless LAN system 10 includes multiple wireless terminals orstations (STAs). A laptop PC can be used as a STA as an example. TheSTAs 20-1, 20-3, and 20-5 present in the exclusive area of the cell 21and the STA 41 present in the cell overlap area are connected to the AP20. The STAs 30-1, 30-3, and 30-5 present in the exclusive area of thecell 31 and the STA 43 present in the cell overlap area are connected tothe AP 30. The STA 45 does not belong to any cell and is not connectedto the wireless LAN system 10. All STAs shown in FIG. 1 possess theSSIDs of the respective APs connected to the wireless LAN system 10 andare registered in the authentication server 13 as authorized clients bytheir identifiers.

Moreover, the authentication server 13 and the respective STAs possess ashared key which is used in authentication and data encryption.Therefore, when connected to any one of the APs, each STA can beauthenticated by the authentication server 13 and can communicatethrough the backbone network. The APs 20 and 30 possess a known DHCPserver (not shown) for assigning an IP address to an STA from which anaccess request is received. There is a limit to the number of IPaddresses assigned by each DHCP server. Moreover, a networkadministrator puts a limit to the number of STAs that can be connectedto each of the APs 20 and 30 at the same time.

Each STA can connect to any one of the APs 20 and 30 as long as it ispresent in the cell area. Therefore, the STA 41 present in the celloverlap area can connect to the AP 30, and the STA 43 can connect to theAP 20. Hitherto, in order to determine an AP to which the STAs 41 and 43present in the cell overlap area are connected, each STA comparesReceived Signal Strength Indications (RSSIs) representing the radiofield intensity of a beacon frame received from the respective APs andselects an AP having a strong radio field intensity as its connectingdestination. If the performances of the respective APs are the same, allgeneral STAs are configured to connect to an AP having a strong radiofield intensity since a high transmission rate can be obtained.

Therefore, in some case, a number of STAs are connected to a specificAP, the total traffic of the respective STAs becomes close to thethroughput of the AP, and the traffic of the respective STAs decreases.Moreover, an STA which enters into a cell exclusive area and newlyconnects to an AP may be unable to connect to the AP since the number ofSTAs connected to the AP is saturated. In the present invention, asdescribed later, an algorithm of allowing the respective STAs toautonomously realize the load balancing of the AP. In this description,it is assumed that the STA 41 is connected to the AP 20, and the STA 43is connected to the AP 30.

A collective entity made up of the AP 20 and the STAs 20-1, 20-3, 20-5,and 41 which have established their logical connection with the AP 20 isreferred to as a basic service set (BSS). Similarly, the AP 30 and theSTAs 31-1, 30-3, 30-5, and 43 forms another BSS. The BSS including theAP 20 will be denoted by a BSS 22, and the BSS including the AP 30 willbe denoted by a BSS 32. An STA that newly participates in any one of theBSSs searches for the SSIDs of connectable APs by an active scanningmethod a passive scanning method. When an STA which is connected to anyone of the APs and is present in the cell exclusive area is movingacross the cells 21 and 31 and is entering into the cell overlap area,the STA is also connectable to other APs.

If the radio field intensity of an AP to which the STA is presentlyconnected weakens when the STA is moving in the cell overlap area, itsconnecting AP is automatically changed while maintaining an existingsession. An operation of the STA changing its connecting AP is calledroaming or handover. When the STA changes its connecting AP, an IPaddress is assigned by a DHCP server of the new AP after change, and theIP address assigned by a DHCP server of the previous AP before change isreleased and can be assigned to other STAs.

FIG. 2 is a diagram showing a network configuration of a wireless LANsystem employing an ad-hoc mode defined by the IEEE 802.11 standards.Each STA can communicated with each other by connecting to the backbonenetwork 11 in the infra mode and can also perform wireless communicationwith each other by constructing an ad-hoc mode network (or an ad-hocnetwork). In the ad-hoc mode, the STAs can communicate directly witheach other without through an AP. A collective entity made up ofmultiple STAs capable of communicating with each other in the ad-hocmode is referred to as an independent basic service set (IBSS). The IBSSis independent from a BSS and can be configured to include STAsbelonging to multiple BSSs.

The ad-hoc network can be constructed by any STA that initiatescommunication with other STAs. The ad-hoc network is formed betweenrespective STAs which are present within the range of radio coverage ofthe STA having constructed the ad-hoc network. FIG. 2 shows an ad-hocnetwork 51 constructed by the STA 20-1 and an ad-hoc network 53constructed by the STA 30-3. The STAs 20-1 and 30-3 which are notpresent in the same ad-hoc network cannot communicate directly with eachother. However, the shared information described later such as theperformance information of APs and the connection information of STAswith respect to the APs can be exchanged through the STA 41 or 43 whichis the member of the two ad-hoc networks 51 and 53.

In the present invention, each STA has a unified communication channeland SSID and possesses a shared key and is thus able to communicate withany STA which is the member of the ad-hoc network. FIG. 2 shows a statein which the STAs 20-1 and 30-3 can communicate with other STAs.However, even when the ad-hoc networks are constructed by differentSTAs, they can communicate with other STAs. The ad-hoc mode employs anactive scanning method whereby a STA which wants to initiate anycommunication transmits a probe request frame and is able to communicatewith a STA having responded with a probe response frame. Moreover, thead-hoc mode employs a passive scanning method whereby each STA receivesa beacon frame from another STA and is able to communicate with the STA.In the present invention, the communication methods in the ad-hoc modeare well known to those having ordinary skill in the art.

B. Configuration of STA

FIG. 3 is a schematic block diagram showing a hardware configuration ofeach STA. A STA 100 is a laptop PC, but is not limited to this and maybe a wireless terminal such as a PDA or a smart phone having the samefunction. The STA 100 includes a central processing unit (CPU) 101, amain memory 103, a disk drive 105, a liquid crystal display (LCD) 107,an input/output device 109, and a wireless controller 111 which areconnected to a bus 115. A program according to the present embodiment isstored in the disk drive 105. An antenna 113 is connected to thewireless controller 111.

The wireless controller 111 operates as a transmitter and a receiver.The wireless controller 111 includes a buffer function whenbidirectionally transmitting IP packets between the bus 115 and awireless medium. The wireless controller 111 includes a protocolconversion function of appending a MAC header to an IP packet duringtransmission to generate an MAC frame and removing the MAC header fromthe MAC frame during reception. The wireless controller 111 alsoincludes a function of encoding modulating data during transmission tooutput the data as high-frequency signals to the antenna 113 and afunction of demodulating signals and performing error correctionprocessing during reception. The functions of the wireless controller111 are well known to those having ordinary skill in the art.

FIG. 4 is a diagram showing the software and hardware configuration forallowing the STA 100 to autonomously adjust the load of an AP. Aconnection utility 155 is software including a well-known function ofperforming processing for connecting the STA 100 to an AP or allowingthe STA 100 to roam to another AP. The connection utility 155 alsoincludes a new function of autonomously adjusting the load of the AP byreferring to a traffic table 157. The traffic table 157 is formed on astorage area on the main memory 103 in order to store the performanceinformation representing the load performance of the respective APs andthe connection information representing the connection state of STAsconnected to an AP. The new function of the connection utility 155 andthe data structure of the traffic table 157 will be described later. Anapplication 159 is a web browser, a mailing software, or the like whichoperates on an operating system (OS) 153 so as to transmit and receivedata to/from the server of the backbone network 11. The application 159forms an application layer, a presentation layer, and a session layer ofthe OSI reference model.

The OS 153 may be Windows 7®. Windows 7 has a function called “VirtualWiFi” in which one wireless controller 111 is virtualized so as tofunction as multiple wireless controllers and a function called “SoftAP” of emulating the function of APs. A device driver 151 is a softwarethat controls the operation of the wireless controller 111 and datatransmission, and under the Windows 7, it can cause the wirelesscontroller 111 to operate in the infra mode and the ad-hoc mode at thesame time.

Therefore, for example, when the application 159 is communicating withthe backbone network 11 using the infra mode of the wireless controller111, the STA 100 can communicate with other STAs in the ad-hoc modewithout disconnecting the session. The STA 100 may use an OS in which aninfra mode wireless controller and an ad-hoc mode wireless controllerare separately mounted, and the virtual function is not included. The OS153 forms a transport layer and a network layer of the OSI referencemodel.

The device driver 151 is capable of measuring traffic which is thequantity of data transmitted per a unit period when the wirelesscontroller 111 transmits and receives data to/from an AP. The connectionutility 155 is capable of receiving periodically the present trafficfrom the device driver 151. The device driver 151 is capable ofmeasuring the radio field intensity (RSSI) and the signal-to-noise ratio(SN ratio) of the beacon frame transmitted by an AP and transmitting themeasured values to the connection utility 155. The RSSI and the SN ratioare parameters of communication quality that determines the transmissionrate between an AP and an STA. The device driver 151 forms a data linklayer of the OSI reference model. The wireless controller 111 forms aphysical layer of the OSI reference model.

The connection utility 155, the traffic table 157, the OS 153, and thedevice driver 151 shown in FIG. 4 are loaded into the main memory 103and executed by the CPU 101, thus realizing various functions incollaboration with the hardware of the STA 100. For example, theconnection utility 155, the OS 153, and the device driver 151 constitutea means for acquiring the performance information of an AP, a means forcommunicating the connection information in the ad-hoc mode to exchangeit with other STAs, or a means for determining a connecting destinationAP based on the performance information and the connection information.These means are not limited to the configuration shown in FIG. 4, and aconfiguration in which other software and hardware or either one of themis added is also included within the scope of the present invention, solong as one having ordinary skill in the art can arbitrarily select theconfiguration.

It should be understood that FIGS. 3 and 4 only illustrate the primaryhardware constructions related to the present embodiment and theconnections in order to describe the present embodiment. In addition tothe components described above, many other devices are used for theconstitution of the STA 100. However, since these are well known to onehaving ordinary skill in the art, detailed explanations for them willnot be provided here. Of course, multiple blocks shown in FIGS. 3 and 4may form a single integrated circuit or device, or one block may bedivided into multiple integrated circuits or devices, and theseconfigurations are also included within the scope of the presentinvention, so long as one having ordinary skill in the art canarbitrarily select the configurations.

C. Data Structure of Traffic Table

FIG. 5 is a diagram showing a data structure of the traffic table 157possessed by the STA 100. FIG. 6 is a diagram illustrating a method ofcalculating an average traffic (unit: bps) corresponding to the averagequantity of data transmitted per a predetermined period of time by theconnection utility 155 from the traffic measured by the device driver151. It is assumed that the STA 100 presently belongs to the BSS 22.

In the traffic table 157, information about the AP 20 of the BSS 22 towhich the STA 100 belongs and the AP 30 of the BSS 32 that forms thecell overlap area with the AP 20 are registered. The informationincludes an SSID, an MAC address, the maximum connectable number, athreshold connectable number, a throughput, and a threshold traffic.These data are directly acquired from an AP or provided in advance torespective STAs by a network administrator, and are stored in the diskdrive 105. Alternatively, these data may be stored in the traffic table157 on the main memory 103 by the STA 100 communicating with other STAsin the ad-hoc mode.

The SSID is a name on a network that the network administrator hasassigned to an AP, and in some cases, it may be referred to as ESSIDconsidering a case where there are multiple APs. The MAC addresscorresponding to the SSID is also referred to as a BSSID, and the STA100 can acquire the MAC address from the beacon frame transmitted by anAP before connecting to the AP. Each STA forms the traffic table 157 onthe main memory 103, and upon receiving the beacon frame from any one ofthe APs, the STA first stores the SSID and the MAC address thereof inthe traffic table 157.

The maximum connectable number, the threshold connectable number, thethroughput, and the threshold traffic are the performance informationrepresenting the performance of an AP and are determined uniquely foreach AP. The performance information of each AP is stored in advance inthe disk drive 105 of each STA so as to be correlated with the MACaddress of the AP. Therefore, the connection utility 155 reads out theperformance information corresponding to the MAC addresses of the APs 20and 30 stored in the traffic table 157 from the disk drive 105 andstores the performance information in the traffic table 157.Alternatively, the STA 100 may receive the performance information fromother STAs by communicating in the ad-hoc mode.

The performance information may be included in the management frame suchas the beacon frame for the passive scanning method or the proberesponse frame for the active scanning method at the time of scanning anAP. Then, each STA may acquire the performance information beforeconnecting to an AP and determine its connecting destination. Since avariable-length field (vendor specific) that can be freely used by avendor is defined in such a management frame, the performanceinformation can be included in the field. By allowing a STA to directlyacquire the performance information from an AP, it is easy to cope witha change of an AP, and it is possible to eliminate a production step forstoring the performance information in advance in each STA.

The maximum connectable number represents the upper limit of the numberof STAs connectable to the AP in order to limit the number of IPaddresses or maintain the traffic of each STA to be within apredetermined range. The threshold connectable number is a referencevalue that is set to be smaller than the maximum connectable number. Thethroughput represents the maximum transmission rate of an AP. Thethreshold traffic is a reference value that is set to be smaller thanthe throughput. The threshold connectable number and the thresholdtraffic are reference values which are set to each AP in order to allowa STA present in the cell overlap area to autonomously determine whichone of the BSSs 22 and 32 it will newly participate in.

The threshold connectable number and the threshold traffic are alsoreference values which are set to the presently participating BSS 22 inorder to allow the STA 100 present in the cell overlap area toautonomously determine whether it will roam from the participating BSS22 to another BSS 32. Furthermore, the threshold connectable number andthe threshold traffic are also reference values which are set to theroaming destination BSS 32 in order to allow the STA 100 present in thecell overlap area to autonomously determine whether it will roam fromthe participating BSS 22 to another BSS 32. Although the thresholdconnectable number and the threshold traffic are used for theabove-mentioned three purposes, they may have different values inaccordance with the purpose.

In the traffic table 157, an average traffic corresponding to the MACaddresses which are identifiers of the respective STAs connected to theAP 20 and an average traffic corresponding to the MAC addresses whichare identifiers of the respective STAs connected to the AP 30 arestored. The identifier and the traffic are the connection informationrepresenting the connection states of the STAs. When the STA 100connects to the AP 20 and is transmit data in response to the request ofthe application 159, the connection utility 155 periodically receives atraffic 161 (FIG. 6) from the device driver 151. Since the unit periodof time when the device driver 151 calculates the traffic is relativelyshort, the traffic has a large variation and is not suitable as data tobe used for calculating the total traffic for determining the size ofload of the AP. The connection utility 155 observes the traffic receivedfrom the device driver 151 for a predetermined period tm and divides thetotal quantity of data transmitted during that period by thepredetermined period tm, thus calculating the average traffic.

The connection utility 155 calculates the average traffic for eachpredetermined period is as a moving average. As an example, thepredetermined period tm is 5 minutes, and the predetermined period ts is1 minute. The STA 100 calculates the moving average traffic for 5minutes every one minute and stores the moving average traffic so as tobe correlated with its own MAC address of its own traffic table 157.Then, each STA notifies other STAs in the ad-hoc mode of its own averagetraffic whenever it is updated. Alternatively, each STA may acquire thelatest average traffic by requesting to other STAs every predeterminedperiod ts.

In the traffic table 157 of FIG. 5, the average traffic calculated bythe STA 100 in this way and the average traffic of other STAs receivedfrom the other STAs are stored. The connection utility 155 calculatesthe total number of STAs presently connected to each AP and the sum ofthe average traffic of the respective STAs with respect to each AP fromthe MAC addresses and the average traffic which the connection utility155 received from the respective STAs connected to the BSSs 22 and 32through communication in the ad-hoc mode and stores the data in thetraffic table 157. In the traffic table 157, for the AP 20 of which thethreshold connectable number is 28 and of which the threshold traffic is700, the present total connected number is 5, and the sum of the averagetraffic is 750. Moreover, for the AP 30 of which the thresholdconnectable number is 23 and of which the threshold traffic is 500, thepresent total connected number is 4, and the sum of the average trafficis 220.

In the traffic table 157, a roaming flag field is provided so as tocorrespond to the MAC address of each STA. When a STA having detectedthat there are multiple APs that can be connectable in the cell overlaparea is connected to an AP having a weak radio field intensity based onthe result of comparison between the threshold connectable number andthe present total connected number or based on the result of comparisonbetween the threshold traffic and the sum of average traffic, the STAsets its MAC address in the radio field. The roaming flag is used forallowing each STA to determine whether it is the subject of roaming whenthe STA later becomes able to perform roaming to an AP having a strongradio field intensity.

When there are three or more connectable APs, in order to specify aroaming destination AP, the identifier of an AP having a strong radiofield intensity to which the STA was unable to connect at the time ofnewly participating in a BSS may be appended to the roaming flag.Moreover, when there are multiple STAs in which the roaming flag is setfor the same AP to which the STAs were unable to connect, in order todetermine the priority of roaming, a time stamp may be appended to theroaming flag so that they can perform roaming in the order that theywere connected to the same AP having a weak radio field intensity. FIG.5 shows a state where when the STA 43 autonomously determines to connectto the AP 30 having a weak radio field intensity since the sum of theaverage traffic of STAs with respect to the AP 20 having a strong radiofield intensity exceeds the traffic threshold, and the STA 43 sets theroaming flag for itself. The performance information and the connectioninformation stored in the traffic table 157 are shared between therespective STAs through the ad-hoc network, they are referred to asshared information.

D. Procedure of Load Adjustment when Newly Participating in BSS

FIG. 7 is a flowchart showing a method in which the STA 45 of FIG. 1determines its own connecting destination AP when newly participating inthe BSS 22 or 32 and adjusts the load of the AP. In block 201, as shownin FIG. 1, the BSSs 22 and 32 are constructed in the wireless LAN system10. In block 203, each STA communicates the shared information stored inthe traffic table 157 shown in FIG. 5 in the ad-hoc mode so as to updatethe shared information periodically. The shared information includes theperformance information of the APs 20 and 30 which construct the ad-hocnetwork and to which the respective STAs are connected and theconnection information of each STA. The connection utility 155 of theSTA 45 which is not connected to any AP receives the beacon framestransmitted by APs and searches for a connectable AP. In block 205, theSTA 45 enters into the cell overlap area of the BSSs 22 and 32.

Although the STA 45 may newly participate in a BSS from the cellexclusive area of the BSS 22 or 32, in that case, since there is no needto select a connecting destination AP, the load adjustment of an AP isnot performed. The APs 20 and 30 transmit a beacon frame periodically.The beacon frame includes information such as the SSID of the AP, anavailable wireless channel, and a supporting transfer rate. Theconnection utility 155 of the STA 45 having moved into the cell overlaparea receives the beacon frames transmitted by the APs 20 and 30 fromthe device driver 151 and recognizes the presence of the connectable APs20 and 30 from their SSIDs. The device driver 151 measures the radiofield intensities of the APs 20 and 30 from the received beacon framesand transmits the measured values to the connection utility 155.

When multiple connectable APs is detected, the connection utility 155 ofthe STA 45 is configured to communicate in the ad-hoc mode to acquirethe shared information stored in the traffic table 157 from any one ofthe STAs before initiating the authentication for an AP having a strongradio field intensity as in the case of the related art. When the MACaddresses are acquired from the beacon frames of the APs 20 and 30, theSTA 45 may read out the performance information of the APs 20 and 30from its own disk drive 105 and store the data in the traffic table.

In block 207, the STA 45 establishes an ad-hoc network, communicateswith any one of the STAs constituting the BSS 22 or 32 to acquire theshared information and store the same in its traffic table 157. Uponacquiring the shared information regarding the APs 20 and 30 that formthe cell overlap area from any one of the STAs to which the STA 45 wantsto connect, the connection utility 155 autonomously determines which APthe STA 45 will connect to in the following order.

Autonomous determination is defined as each STA makes its own decisionbased on only the shared information in the traffic table 157 withoutthe support from a server or an AP connected to the backbone network 11.

The STA 45 determines that the radio field intensity of the beacon frametransmitted by the AP 20 is stronger than that of the AP 30. In block209, the connection utility 155 first determines whether it is possibleto connect to the AP 20 based on the threshold connectable number andthe present total connected number of the AP 20 having a strong radiofield intensity by referring to the traffic table 157. When the presenttotal connected number is equal to or smaller than the thresholdconnectable number, the flow proceeds to block 211. In block 211, theconnection utility 155 of the STA 45 determines whether or not the sumof the average traffic of the STAs connected to the AP 20 is equal to orsmaller than the threshold traffic by referring to the traffic table157. When the sum of the average traffic is equal to or smaller than thethreshold traffic, the STA 45 determines that it is possible to connectto the AP 20 considering the load balance between the APs 20 and 30, andthen, the flow proceeds to block 213. The STA 45 connects to the AP 20in block 213, stores its own average traffic in the traffic table inblock 215 and exchange the shared information with other STAs throughcommunication in the ad-hoc mode.

When it is determined in block 209 that the present total connectednumber exceeds the threshold connectable number or when it is determinedin block 211 that the sum of the average traffic exceeds the thresholdtraffic, the STA 45 determines that it is not possible to connect to theAP 20 considering the load balance between the APs 20 and 30, and theflow proceeds to block 219. In block 219, the connection utility 155determines whether or not it is possible to connect to the AP 30 havinga weak radio field intensity by referring to the traffic table 157. Whenthe present total connected number of the AP 30 is equal to or smallerthan the threshold connectable number, the flow proceeds to block 221.In block 221, the connection utility 155 of the STA 45 determineswhether or not the sum of the average traffic of the STAs connected tothe AP 30 is equal to or smaller than the threshold traffic by referringto the traffic table 157. When the sum of the average traffic of therespective STAs connected to the AP 30 is equal to or smaller than thethreshold traffic, the STA 45 determines that it is possible to connectto the AP 30 considering the load balance, and the flow proceeds toblock 223.

When it is determined in block 219 that the present total connectednumber of the AP 30 exceeds the threshold connectable number, or when itis determined in block 221 that the sum of the average traffic of the AP30 exceeds the threshold traffic, the flow proceeds to block 213. Theconnection condition of the AP 20 is satisfied when the two conditionsof blocks 209 and 211 are satisfied, and the connection condition of theAP 30 is satisfied when the two conditions of blocks 219 and 221 aresatisfied. When the two connection conditions of the APs 20 and 30 arenot satisfied, the STA 45 connects to the AP 20 in block 213. Even whenthe connection condition of the AP 20 is not satisfied, if the presenttotal connected number is equal to or smaller than the maximumconnectable number, and the sum of the average traffic is equal to orsmaller than a throughput, it is possible to connect the STA 45 to theAP 20 although the respective STAs experience a decrease of traffic as awhole. When the two connection conditions of the APs 20 and 30 are notsatisfied, it is preferable to connect the STA 45 to the AP 20 having astrong radio field intensity from the perspective of service rather thannot connecting it to any of the APs 20 and 30.

According to the shared information in the traffic table 157 shown inFIG. 5, since the sum of the average traffic of the respective STAsconnected to the AP 20 exceeds the threshold traffic, the connectioncondition of the AP 20 is not satisfied. On the other hand, since thepresent total connected number of the AP 30 is equal to or smaller thanthe threshold connectable number, and the sum of the average traffic isequal to or smaller than the threshold traffic, the connection conditionof the AP 30 is satisfied. Therefore, the STA 45 autonomously determinesthat the AP 30 is its connecting destination AP. In block 223, theconnection utility 155 of the STA 45 initiates connection to the AP 30.First, the STA 45 sends an authentication request by sending amanagement frame including its own MAC address to the AP 30. The AP 30responds by performing authentication on the MAC layer such as sharedkey authentication or open authentication. Subsequently, the STA 45sends an association request by sending a management frame to the AP 30.

When the STA 45 and the AP 30 set their communication parameters tocomplete association, the DHCP server of the AP 30 assigns an IP addressto the STA 45. After that, the AP 30 asks the authentication server 13for authentication of the STA 45, whereby the STA 45 is authenticated bythe authentication server 13. When an encryption key used between the AP30 and the STA 45 is sent from the authentication server 13 and set, theSTA 45 is able to perform data communication with the backbone network11, and its participation in the BSS 32 is finished. When the STA 45 isconnected to the AP 30, the STA 45 sets a roaming flag in its owntraffic table 157.

The application 159 of the STA 45 participating in the BSS 32 initiatesdata communication with a Web server connected to the backbone network11 or other STAs connected to the AP 30. In block 225, the connectionutility 155 receives the traffic information from the device driver 151,calculates the average traffic for the predetermined period tm, andstores the measured value in its traffic table 157 every predeterminedperiod ts. As in the case of block 203, multiple STAs including the STA45 exchange the shared information by periodically communicating in thead-hoc mode.

When the present total connected number of STAs connected to the AP 20is equal to or smaller than the maximum connectable number, the STA 45is connected to the AP 20 having a strong radio field intensity. As aresult, when the present total connected number of the AP 20 reaches themaximum connectable number, even if another STA of the BSS 22 is able toparticipate in the BSS 32, STAs will be unable to participate in the BSS22 from the cell exclusive area of the AP 20. Moreover, due to theparticipation of the STA 45, the sum of the average traffic of the AP 20reaches or exceeds the throughput of the AP 20, whereby the traffic ofthe STAs constituting the BSS 22 decreases as a whole. However,according to the above-described procedure, when the STA 45 determinesthat the AP 20 has a smaller margin than the AP 30 with respect to thepresent total connected number or the sum of the average traffic, sincethe STA 45 connects to the AP 30 having the larger margin, theabove-described problem does not occur.

The conditions of blocks 209 and 211 correspond to the connectioncondition of the AP 20 having a strong radio field intensity, and theconditions of blocks 219 and 221 correspond to the connection conditionof the AP 30 having a weak radio field intensity. When the twoconnection conditions of the AP 20 having a strong radio field intensityare satisfied, the STA 45 connects to the AP 20 having a strong radiofield intensity. When one of the two connection conditions of the AP 20having a strong radio field intensity is dissatisfied, and the twoconnection conditions of the AP 30 having a weak radio field intensityare satisfied, the STA 45 connects to the AP 30 having a weak radiofield intensity. In some cases, the total connected number or the sum ofthe average traffic of the BSS 22 changes after the STA 45 is connectedto the AP 30 having a weak radio field intensity.

Since the STA 45 can experience a higher transmission rate when itconnects to the AP 20 having a strong radio field intensity unless itmoves from the present position, when the AP 20 has a margin, it ispreferable for the STA 45 to detect such a margin and perform roaming tothe AP 20. In block 225, the STA 45 having the roaming flag set thereinrecognizes that it is the subject of roaming. When the roaming acceptingcondition of the AP 20 is satisfied, the STA 45 autonomously determinesto perform roaming. The roaming accepting condition is satisfied whenthe total connected number of the AP 20 having a strong radio fieldintensity is equal to or smaller than the threshold connectable number,and the sum of the average traffic is equal to or smaller than thethreshold traffic.

The roaming accepting condition is not related to the total connectednumber or the sum of the average traffic of the roaming originating AP30 having a weak radio field intensity. When the roaming acceptingcondition is not satisfied, the flow returns to block 223, and the STA45 maintains the connection to the AP 30. When the roaming acceptingcondition is satisfied, the flow proceeds to block 213, and the STA 45changes its connecting destination AP from the AP 30 to the AP 20. Theroaming as mentioned herein is different from normal roaming whichoccurs when the STA 45 moves so that the radio field intensity of the AP30 weakens. In contrast, even when the STA 45 does not move, the roamingas mentioned herein occurs in accordance with changes in both or eitherone of the total connected number and the sum of the average traffic ofthe roaming destination AP 20 having a strong radio field intensity.

In the description above, although a case where the cell overlap area isformed by two cells has been described as an example, theabove-described procedure can be applied to a case where the celloverlap area is formed by three or more cells. In this case, the STArepeats the procedures of determining as to the connection conditions inblocks 209 and 211, and 219 and 221, of the APs in descending order ofthe radio field intensity by referring to the traffic table 157, selectsan AP having the greatest radio field intensity among the APs satisfyingthe connection conditions by its own decision, and connects to theselected AP.

The threshold connectable number and the threshold traffic used in theconnection conditions of blocks 209 and 211 or blocks 219 and 221 mayhave different values from those of the threshold connectable number andthreshold traffic used in the roaming accepting condition used in block227. In this case, it is preferable to take measures as necessary sothat the threshold value of the roaming accepting condition is smallerthan the threshold value of the connection condition, thus making theroaming condition stricter than the connection condition. This is toprevent the margin in the performance and load of the two APs from beingreversed by the roaming.

E. Procedure of Load Adjustment after Having Participated in BSS

In multiple BSS that shares a cell overlap area, it is ideal to maximizethe traffic of the respective STAs constituting all multiple BSS as awhole. Moreover, it is preferable that all the BSS can take measures soas to prevent the occurrence a situation in which a STA participating ina BSS from a cell exclusive area is unable to connect to the BSS due tothe limitation on the total connected number of an AP. On the otherhand, an STA having already connected to any one of the APs generallywill not disconnect its connection to the AP unless the STA stopsoperating or moves out of the cell coverage even when it does notperform wireless communication. Moreover, the traffic of the respectiveSTAs connected to an AP changes with time in accordance with a change ofthe user's work, and an unbalance of load such that one BSS has a largeload and the other BSS has a small load may occur. The STA according tothe present invention can autonomously solve such a problem by using theshared information.

Detailed description will be provided by way of the wireless LAN system10 shown in FIG. 1. If the radio field intensity of the AP 20 isstronger than the AP 30, and the sum of the average traffic of the AP 20is sufficiently smaller than the throughput, it can be said that it ispreferable for the STA 41 present in the cell overlap area of the BSSs22 and 32 to connect to the AP 20 since the transmission rate can beimproved. However, if the sum of the average traffic of the AP 20 getsclose to the throughput, the average traffic of the respective STAsconstituting the BSS 22 decrease as a whole. In this case, if the sum ofthe average traffic of the AP 30 is sufficiently smaller than thethroughput, by allowing the STA 41 to perform roaming to the AP 30, itis possible to improve the traffic of the BSS 22 without causing adecrease in the traffic of the respective STAs of the BSS 32. Moreover,if thirty STAs are connected to the BSS 22, and the STA 41 roams to theAP 30 so that the total connected number of the BSS 22 becomes 29, theSTA 45 can newly participate in the BSS 22 from the cell exclusive area.

FIG. 8 is a flowchart showing a method in which a STA having alreadyparticipated in a BSS autonomously performs roaming and adjusts the loadof an AP. In this method, as an example, the STA 41 autonomouslydetermines whether or not to perform roaming based on the performanceinformation and the connection information of the AP 20 to which the STA41 is presently connected and the performance information and theconnection information of the AP 30 to which the STA 41 can roam. Normalroaming occurs when the STA 41 moves so that the radio field intensityof the presently connected AP 20 decreases. The roaming in thisprocedure is performed based on the sum of the average traffic or thepresent total connected number of the BSS 22 regardless of the movementof the STA 41. In block 301, as shown in FIG. 1, the BSSs 22 and 32 areconstructed in the wireless LAN system 10. In block 303, the respectiveSTAs constituting the BSSs 22 and 32 periodically exchange the sharedinformation and store the shared information in their traffic table 157.

In block 305, the STA 41 determines whether or not the sum of theaverage traffic of its connected AP 20 exceeds the threshold traffic byreferring to the traffic table 157. When the sum exceeds the thresholdtraffic, the flow proceeds to block 309 since there is a possibilitythat the traffic of the respective STAs decrease if the sum of theaverage traffic of the AP 20 increases further.

When the sum of the average traffic is equal to or smaller than thethreshold traffic, the flow proceeds to block 307, and the STA 41determines whether or not the present total connected number of itsconnected AP exceeds the threshold connectable number or reaches themaximum connectable number. When the present total connected numberexceeds the threshold connectable number or reaches the maximumconnectable number, the flow proceeds to block 309. When the presenttotal connected number is equal to or smaller than threshold connectablenumber or smaller than the maximum connectable number, the flow returnsto block 303, and the STA 41 does not perform roaming until any one ofthe conditions of blocks 305 and 307 is satisfied. The conditions ofblocks 305 and 307 are the roaming conditions set for the roamingoriginating BSS 22, and when any one of the conditions is satisfied, theroaming condition of the AP 20 is satisfied.

In block 309, the STA 41 determines whether or not it has a roamable APto which it can roam. A STA which possesses a roamable AP is a STA whichis present in the cell overlap area and which has detected the SSIDs ofmultiple connectable APs. Since the STA 41 has the roamable AP 30, theflow proceeds to block 311. If all STAs are present in the cellexclusive areas and do not possess the roamable AP, no STA performsroaming from the BSSs 22 and 32, and the flow returns to block 303.Then, the respective STAs make determination as to the roaming conditionagain after updating the shared information.

In the wireless LAN system 10 of FIG. 1, although the STAs 41 and 43 arepresent in the cell overlap area and can perform roaming, it is assumedthat the BSS 22 satisfies the roaming condition of the roamingoriginating BSS whereas the BSS 32 does not satisfy the roamingcondition. In block 311, the STA 41 (hereinafter referred to as aroamable STA) which is connected to the AP 20 satisfying the roamingcondition and which possesses a roamable AP determines whether or not itis the STA that performs the current roam. When there is one roamableSTA, the STA determines that it is the STA that performs the currentroaming.

Since the BSS 22 has only one roamable STA 41, the STA 41 determinesthat it is the subject of roaming. When there is multiple roamable STAs,the connection utility 155 of each STA autonomously determines whetherit is the subject of roaming by referring to the traffic table 157. Whenthe roaming condition of block 305 regarding the sum of the averagetraffic is satisfied, a STA having the most average traffic among themultiple roamable STAs may determine that it is the subject of roamingin order to decrease the sum of the average traffic.

Alternatively, a predetermined number of STAs as counted in ascendingorder of the average traffic among the multiple roamable STAs may determine that they are the subjects of roaming. In the case of the formermethod, the corresponding STA is best effective for decreasing thetraffic of the AP, whereas the latter method provides advantages in thatthe corresponding STA has little influence on the interruption oftraffic due to the roaming. When the roaming condition of block 307regarding the present total connected number is satisfied, it ispreferable that a STA having the smallest average traffic among themultiple roamable STAs may determine that it is the subject of roamingin order to decrease the total connected number without affecting thetraffic of a roaming STA.

For the roaming to be effective in adjusting the load of an AP, it isnecessary to ensure that the roaming has little influence on thethroughput and the maximum connectable number of a roaming destinationAP. In block 313, the STA 41 having determined to be the subject ofroaming determines whether or not the roaming accepting condition of aroamable AP is satisfied. The roaming accepting condition is satisfiedwhen the present total connected number of the roamable AP 30 is equalto or smaller than the threshold connectable number, and the sum of theaverage traffic is equal to or smaller than the threshold traffic. Whenthere is no BSS satisfying the roaming accepting condition, the STA 41does not perform roaming.

The roaming condition and the roaming accepting condition are determinedbased on the comparison between the threshold traffic and the sum of theaverage traffic and the comparison between the threshold connectablenumber and the present total connected number. Here, it is preferablethat the threshold traffic and the threshold connectable number of theroaming condition are set to be different from those of the roamingaccepting condition so that the relationship of the performanceinformation and the connection information in the roaming originatingsource and the roaming destination, namely the relationship of themargin in load is not reversed by the roaming. For example, when theperformances of the roaming originating AP and the roaming destinationAP are the same, by setting the threshold traffic and the thresholdconnectable number of the roaming destination so as to be smaller thanthose of the roaming originating source, it is possible to reduce theinfluence on STAs belonging to the roaming destination AP although theload on the roaming destination AP is increased by the roaming. When theroaming originating AP and the roaming destination AP have differentperformances, by setting the threshold traffic and the thresholdconnectable number considering the performances of the respective APs,it is possible to prevent the load margin from being reversed.

In block 315, a roaming destination AP is determined. When there is onlyone AP that satisfies the roaming accepting condition, for example theAP 30 as shown in FIG. 1, the connection utility 155 of the STA 41determines the AP 30 as the roaming destination. When there is multipleAPs that satisfy the roaming accepting condition, the connection utility155 needs to select any one of the APs.

When the roaming originating AP satisfies the roaming condition of block305 regarding the sum of the average traffic, and the STA having themost average traffic is determined to be the subject of roaming in block311, an AP which has the greatest radio field intensity, has the bestcommunication quality as measured by the SN ratio, or has the greatestdifference between the threshold traffic and the sum of the averagetraffic can be determined as the roaming destination AP. Moreover, whenthe roaming originating AP satisfies the roaming condition of block 307regarding the present total connected number, and the STA having thesmallest average traffic is determined to be the subject of roaming inblock 311, an AP which has the greatest difference between the thresholdconnectable number and the present total connected number can bedetermined as the roaming destination AP.

When the STA 41 having determined to perform roaming selects the AP 30as the roaming destination AP, the connection utility 155 of the STA 41changes the wireless channel frequency to change the connectingdestination from the AP 20 to the AP 30 while maintaining the presentsession with the AP 20 by a well-known procedure in block 317. Followingthe roaming, the DHCP server of the AP 20 withdraws the IP addressassigned to the STA 41 in order to assign the IP address to a newparticipating STA. Moreover, the DHCP server of the AP 30 assigns a newIP address to the STA 41.

When roaming, the STA 41 communicates with other STAs in the ad-hoc modeto declare that it is going to perform roaming, whereby it is possibleto securely prevent the occurrence of a situation in which other STAsdetermine that they are the subjects of roaming based on the subsequentchanges in average traffic. Moreover, the STA 41 may make a finaldecision on roaming based on the latest shared information acquired fromother STAs in the ad-hoc mode immediately before roaming. When the STA41 has roamed from the AP 20 to the AP 30, the sum of the averagetraffic of the BSS 22 decreases, whereby the respective remaining STAscan maintain communication using a predetermined traffic, and it ispossible to ensure the connection of a STA that newly participates inthe BSS 22 from the cell exclusive area.

As has been described, the present invention provides a method forallowing a wireless portable computer to adjust the load of a wirelessbase station autonomously.

Those skilled in the art will appreciate that the mechanisms of thepresent invention are capable of being distributed as a computer programproduct in a variety of computer-readable storage medium such as compactdiscs, digital versatile discs, etc.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

1. A method for a laptop computer to perform a wireless connection to awireless base station, said method comprising: acquiring performanceinformation representing a performance of a plurality of wireless basestations that form a cell overlap area to which a plurality of wirelessterminals is connected; before connecting to any one of said wirelessbase stations in said cell overlap area, communicating in a ad-hoc modewith said wireless terminals to receive connection informationrepresenting respective connection states of said wireless terminals tosaid wireless base station, wherein said laptop computer is capable ofcommunicating in an infrastructure mode and said ad-hoc mode to allowsaid laptop computer to connect to any one of said wireless basestations; selecting one of said wireless base station as a candidate formaking a wireless connection based on said performance information andsaid connection information; and connecting said laptop computer to saidselected wireless base station.
 2. The method of claim 1, wherein saidacquiring further includes acquiring said performance information fromrespective wireless base stations through a management frame.
 3. Themethod of claim 1, wherein said acquiring further includes acquiringsaid performance information by communicating in said ad-hoc mode withother wireless terminals before connecting to any one of said wirelessbase stations in said cell overlap area.
 4. The method of claim 1,wherein said selecting is performed based on said performanceinformation and the total traffic of respective wireless terminals withrespect to a specific wireless base station.
 5. The method of claim 1,wherein said connection information includes identifiers of saidwireless terminals and the traffic of respective wireless terminals tosaid wireless base stations.
 6. The method of claim 1, wherein saidmethod further includes providing a traffic table to store saidperformance information and said connection information.
 7. The methodof claim 6, wherein said method further includes periodically storingits own connection information in said traffic table.
 8. The method ofclaim 1, wherein when said laptop computer is connected to a wirelessbase station having a weaker radio field intensity among said wirelessbase stations based on said performance information and said connectioninformation, periodically communicating in said ad-hoc mode with otherwireless terminals connected to a wireless base station having astronger radio field intensity to acquire said connection information ofsaid other wireless terminals; determining whether or not a roamingaccepting condition is satisfied based on said performance informationof said wireless base station having a stronger radio field intensityand said connection information of respective wireless terminalsconnected to said wireless base station having a stronger radio fieldintensity; and roaming to said wireless base station having a strongerradio field intensity when said roaming accepting condition issatisfied.
 9. A computer-readable storage medium having a computerprogram product for wireless connecting a laptop computer to a wirelessbase station, said computer-readable storage medium comprising: programcode for acquiring performance information representing a performance ofa plurality of wireless base stations that form a cell overlap area towhich a plurality of wireless terminals is connected; program code for,before connecting to any one of said wireless base stations in said celloverlap area, communicating in a ad-hoc mode with said wirelessterminals to receive connection information representing respectiveconnection states of said wireless terminals to said wireless basestation, wherein said laptop computer is capable of communicating in aninfrastructure mode and said ad-hoc mode to allow said laptop computerto connect to any one of said wireless base stations; program code forselecting one of said wireless base station as a candidate for making awireless connection based on said performance information and saidconnection information; and program code for connecting said laptopcomputer to said selected wireless base station.
 10. Thecomputer-readable storage medium of claim 9, wherein said program codefor acquiring further includes program code for acquiring saidperformance information from respective wireless base stations through amanagement frame.
 11. The computer-readable storage medium of claim 9,wherein said program code for acquiring further includes program codefor acquiring said performance information by communicating in saidad-hoc mode with other wireless terminals before connecting to any oneof said wireless base stations in said cell overlap area.
 12. Thecomputer-readable storage medium of claim 9, wherein said program codefor selecting is performed based on said performance information and thetotal traffic of respective wireless terminals with respect to aspecific wireless base station.
 13. The computer-readable storage mediumof claim 9, wherein said connection information includes identifiers ofsaid wireless terminals and the traffic of respective wireless terminalsto said wireless base stations.
 14. The computer-readable storage mediumof claim 9, wherein said computer-readable storage medium furtherincludes program code for providing a traffic table to store saidperformance information and said connection information.
 15. Thecomputer-readable storage medium of claim 14, wherein saidcomputer-readable storage medium further includes program code forperiodically storing its own connection information in said traffictable.
 16. The computer-readable storage medium of claim 9, wherein saidcomputer-readable storage medium further includes when said laptopcomputer is connected to a wireless base station having a weaker radiofield intensity among said wireless base stations based on saidperformance information and said connection information, program codefor periodically communicating in said ad-hoc mode with other wirelessterminals connected to a wireless base station having a stronger radiofield intensity to acquire said connection information of said otherwireless terminals; program code for determining whether or not aroaming accepting condition is satisfied based on said performanceinformation of said wireless base station having a stronger radio fieldintensity and said connection information of respective wirelessterminals connected to said wireless base station having a strongerradio field intensity; and program code for roaming to said wirelessbase station having a stronger radio field intensity when said roamingaccepting condition is satisfied.
 17. A laptop computer comprising: awireless controller for acquiring performance information representing aperformance of a plurality of wireless base stations that form a celloverlap area to which a plurality of wireless terminals is connected;before connecting to any one of said wireless base stations in said celloverlap area, for communicating in a ad-hoc mode with said wirelessterminals to receive connection information representing respectiveconnection states of said wireless terminals to said wireless basestation, wherein said laptop computer is capable of communicating in aninfrastructure mode and said ad-hoc mode to allow said laptop computerto connect to any one of said wireless base stations; a memory having atraffic table for storing said performance information and saidconnection information; a processor for selecting one of said wirelessbase station as a candidate for making a wireless connection based onsaid performance information and said connection information; andwherein said wireless controller connects said laptop computer to saidselected wireless base station.
 18. The laptop computer of claim 17,wherein said wireless controller acquires said performance informationby communicating in said ad-hoc mode with other wireless terminalsbefore connecting to any one of said wireless base stations in said celloverlap area.
 19. The laptop computer of claim 17, wherein saidprocessor performs said selection based on said performance informationand the total traffic of respective wireless terminals with respect to aspecific wireless base station.
 20. The laptop computer of claim 17,wherein said connection information includes identifiers of saidwireless terminals and the traffic of respective wireless terminals tosaid wireless base stations.